Controversial influence of aqueous treatments on historic textiles
Introduction
The aqueous immersion of historic textiles with the aim of removing stains and yellowing has been a controversial topic for textile conservators, especially when it comes to the addition of bleaching agents to boost the cleaning and brightening action of the treatment. Particularly oxidative bleaching is considered to be a process that is difficult to control and that might have negative and unpredictable long-term effects on the treated textile [1].
To overcome these shortcomings of oxidative bleaching, sodium borohydride, a reducing bleaching agent, has been introduced into the community of textile conservators about 25 years ago [2]. While oxidative bleaching agents, such as hydrogen peroxide, attack and cleave double bonds and convert aldehydes to carboxylic acid functions, reducing agents such as sodium borohydride convert oxidized cellulose functionalities back to the original hydroxyl groups rather than causing further oxidation [3], [4]. This action was found to be beneficial for the long-term stability of textiles, and researchers have found higher DP and better tear resistance of the treated material after accelerated aging [5], [6].
Borohydride has first been used in the paper industry as a bleaching agent to improve brightness stability [7], [8]. In the 1980s, borohydride was applied to textile objects and has been thoroughly studied afterwards. Generally, authors agree upon the beneficial impact of borohydride treatments on textiles [3], [5], [9], [10].
Apart from the obvious benefits of the treatment with reducing borohydride, several researchers have also mentioned major drawbacks that have kept conservators from using this agent more frequently. The development of hydrogen bubbles in the course of the treatment leads to mechanical stress on weakened textile fibres. Also, the optimum treatment conditions for borohydride require a pH of 9–10. Under such alkaline conditions, oxidized cellulose present in historic textiles, might suffer from β-elimination reactions that will cleave the cellulose molecule wherever a carbonyl group along the cellulose chain had been introduced before by oxidative processes. Last but not least, sodium borohydride is toxic and flammable. Thus, caution has to be exercised while using and discarding it [11].
It has been suggested that neutralisation and removal of water-soluble acids from the objects before the reductive treatment would minimize decomposition of borohydride [3], [6], [12]. Also careful visual examination of the object to be treated for metal traces is strongly recommended as metal ions will catalyze the decomposition of borohydride. This enhances mechanical stress due to increased hydrogen bubbling and diminishes the reactivity of the borohydride as it is consumed by the metal traces rather than by the reduction of oxidized cellulose functionalities [13]. Descriptions of experiments with buffered borohydride solutions at a pH of 7.5 proved that the reductive action of borohydride has been eliminated completely, rendering the treatment procedure useless [6].
Borohydride bleaching is commonly applied in a bath. Literature data of the concentration of the borohydride range between 0.02 and 0.05 mol/l [6]. There are contradictory findings as to whether higher concentrations improve the achieved brightness of the object or not [6], [14]. The development of bubbles, however, is less pronounced at lower concentrations. If longer treatment time is a viable option, lower concentrations may be used to achieve a comparable bleaching effect [5]. It was found that the bleaching process is rather slow [3], but the biggest impact on brightness is observed in the first 15 min [13].
The present study aims at elucidating the impact of washing on different historic textiles in comparison to a reductive borohydride treatment. Next to pure sodium borohydride additional treatments with two aqueous buffering systems were included.
Modern cotton and linen samples (about 100 years old), Egyptian textiles (ca. 1000 BC), and Inca textiles (ca. 900 AD) were used to complete the aspect of washing on differently aged cloths. All Inca textiles were made out of local cotton fibres. They originate from the Lambayecque culture, a region in northwestern Peru that is known for its rich historical past. The name originates from an ancient pre-Inca civilization.
Section snippets
Labelling
Carbazole-9-carbonyloxyamine (CCOA) labelling of carbonyl groups was performed as described earlier [15], [16], [17]. Fluorenyl diazomethane (FDAM) labelling of carboxyl groups followed the protocol of Bohrn et al. [18].
General analytical methods
Gel permeation chromatography (GPC) measurements used the following components: online degasser, Dionex DG-2410; Kontron 420 pump, pulse damper; auto sampler, HP 1100 column oven, Gynkotek STH 585, fluorescence detector TSP FL2000 (CCOA) and Shimadzu RF535 (FDAM); multiple-angle
Washing
Washing is thought to remove cellulose degradation products of low-molecular weight, such as monosaccharides, oligosaccharides, sugar acids, hydroxy acids, and others [19]. Additionally, some brightening of the textile is usually achieved as dirt and some stains are taken out of the textile. The removal of such contaminants, however, should not influence the molecular weight of the remaining cellulose that is by far too high to allow water solubility.
The washing of ancient Inca textiles had two
Discussion
The present study aimed at the analysis of the impact of aqueous washing and reductive bleaching treatments on cellulosic textiles which possibly had a high degree of oxidative damage. Especially the reductive treatment option, namely sodium borohydride, is reported to improve the long-term preservation of the treated textiles. However, as the reduction by means of sodium borohydride takes place in an alkaline environment, chain scission due to β-elimination reactions starting from carbonyls
Conclusions
The effect of washing on historic textile is difficult to predict. However, in general a beneficial effect was shown in most cases, which is due to the fact that degraded, lower molecular weight fractions were removed from the textile upon washing in a water bath. The treatment duration appeared to have an important influence on the result: rather short treatments as performed with the Inca textiles led to better results than extended treatments, as shown for the historic cotton textiles that
Acknowledgements
The authors would like to thank Karin von Lerber for fruitful discussions of the results. We thank Dr. Zuckerstätter, Wood Kplus, Lenzing, Austria, for performing the CP-MAS NMR measurements.
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